1. Field of the Invention
The present invention relates to an ink-jet printhead and a manufacturing method thereof, and more particularly, to an ink-jet printhead using a shape memory alloy and a manufacturing method thereof.
2. Description of the Related Art
In general, an ink-jet printhead is a device printing a predetermined color image on a recording sheet by ejecting a small volume of a droplet of printing ink at a desired position on the recording sheet and generally utilizes a drop on demand (DOD) system injecting the small volume of the droplet of ink on the recording sheet only on demand.
An ink ejection mechanism of the ink-jet printhead using the DOD system includes a heating-type ejecting method of ejecting ink by generating a bubble in ink using a heat source, a vibrating-type ejecting method of ejecting ink by a volume variation of ink caused by a deformation of a piezoelectric device, and an ejecting method using a shape memory alloy to eject ink by the volume variation of ink, which is caused by the shape memory alloy returned to a memorized original state.
In terms of the heating-type ejecting method, a quite great electric power is supplied to a heater within a very short time to supply heat to a chamber of the ink-jet printhead. The heat is generated by the heater having a specific resistance. Heat is transferred from the heater to the ink through an insulating layer contacting ink, and thus a temperature of water-soluble ink rapidly increases over a critical point. Bubbles are formed when the temperature of the water-soluble ink increases over the critical point, and the bubbles push ink corresponding to a volume of bubbles, thereby applying a pressure to circumferential ink. Ink is ejected from a nozzle in response to kinetic energy by the pressure and the volume variation. The ink forms the ink droplet, and the droplet is ejected onto the recording sheet so as to minimize a natural surface energy of the ink.
The heating-type ejecting method involves a difficulty in maintaining a durability of the printhead due to a successive shock caused by the pressure generated when the bubble generated by a thermal energy is destroyed (burst), and in regulating a size of the ink droplet.
In terms of the vibrating-type ejecting method, ink is pushed by applying a pressure to a chamber using piezoelectric characteristics, which cause a force generated when a voltage is applied to a piezoelectric material attached to a diaphragm, to apply pressure to the chamber of the ink-jet printhead.
The ink-jet printhead using the vibrating-type ejecting method is high in cost due to the use of a high-priced piezoelectric element. In addition, since the piezoelectric element must be harmonized with an electrode, an insulating layer, and a protection layer, an ink-jet printhead manufacturing process becomes complicated, and thus yield of the ink-jet printhead decreases.
FIG. 1 is a cross-sectional view of a conventional ink-jet printhead using a shape memory alloy disclosed in U.S. Pat. No. 6,130,689.
Referring to FIG. 1, an ink-jet printhead includes a substrate 10 having a space portion 11, which penetrates therethrough in up and down directions, a vibration plate 12 (12a, 12b) jointed to an upper portion of the substrate 10 to cover the space portion 11, an electrode 21a having one side contacting the vibration plate 12 to supply current to the vibration plate 12, a nozzle plate 18 installed on the substrate 10, in which a nozzle 19 is formed to eject ink 20, a passage plate 13 disposed between the substrate 10 and the nozzle plate 18 in which an ink chamber 14 is formed to store the ink 20, and a passage 16 providing a path through which the ink 20 flows to the ink chamber 14.
In the ink-jet printhead having the above structure, as shown in FIG. 1, the vibration plate 12 is deformed by a residual stress and is bent toward the space portion 11. If current is applied to the vibration plate 20 through the electrode 21a from an outside source, the vibration plate 12 moves toward the nozzle plate 18 and then is evenly returned to an original state. Here, a volume of the ink chamber 14 is changed, and the ink 20 is ejected onto a printing sheet from the nozzle 19 by the kinetic energy.
In the ink-jet printhead using the shape memory alloy, a resistivity of the shape memory alloy is less than half of a conventional heating element, and thus a large amount of power must be supplied. In particular, the resistivity is changed, for example, from 70-80 μΩ·cm to 100-120 μΩ·cm when the shape memory alloy is changed from a martensite phase to an austenite phase. Accordingly, a variation range of the supplied power increases, and it becomes difficult to precisely control the supplied power. When the supplied power is not precisely controlled, an amount of the ejected ink cannot be precisely regulated, thereby having no predetermined image quality.
In addition, since the shape memory alloy directly contacts the ink, the current flows directly to the ink from the electrode, and thus a composition of the ink is changed, and a desired ejection of the ink cannot be achieved.